add Subarray Sum Equals K using prefix sum

[ruturajjadhav07]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

Subarray Sum Equals K

This PR adds a new algorithm to the prefixsum package:

• SubarraySumEqualsK.java — Counts the number of continuous subarrays that sum to a target value k using prefix sum + HashMap technique in O(N) time.
• SubarraySumEqualsKTest.java — JUnit 5 tests covering positive numbers, negative numbers, zeros, empty arrays, and null input.

References:

• Prefix Sum (Wikipedia)
• LeetCode 560

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.38%. Comparing base (dfaa495) to head (964175f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7252      +/-   ##
============================================
+ Coverage     79.36%   79.38%   +0.01%     
- Complexity     7012     7017       +5     
============================================
  Files           784      785       +1     
  Lines         22970    22982      +12     
  Branches       4519     4520       +1     
============================================
+ Hits          18231    18244      +13     
  Misses         4017     4017              
+ Partials        722      721       -1     

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